Allow using OpenAI models for search in Khoj

- Init processor before search to instantiate `openai_api_key'
  from `khoj.yml'. The key is used to configure search with openai models

- To use OpenAI models for search in Khoj
  - Set `encoder' to name of an OpenAI model. E.g text-embedding-ada-002
  - Set `encoder-type' in `khoj.yml' to `src.utils.models.OpenAI'
  - Set `model-directory' to `null', as online model cannot be stored on disk

src/utils/models.py

@@ -0,0 +1,28 @@
+# External Packages
+import openai
+import torch
+from tqdm import trange
+
+# Internal Packages
+from src.utils.state import processor_config
+
+
+class OpenAI:
+    def __init__(self, model_name, device=None):
+        self.model_name = model_name
+        openai.api_key = processor_config.conversation.openai_api_key
+        self.embedding_dimensions = 1536  # Default to embedding dimensions of text-embedding-ada-002 model
+
+    def encode(self, entries, device=None, **kwargs):
+        embedding_tensors = []
+        for index in trange(0, len(entries)):
+            try:
+                response = openai.Embedding.create(input=entries[index], model=self.model_name)
+                embedding_tensors += [torch.tensor(response.data[0].embedding, device=device)]
+                self.embedding_dimensions = len(response.data[0].embedding)  # Set embedding dimensions to this model's
+            except Exception as e:
+                print(f"Failed to encode entry {index} of length: {len(entries[index])}\n\n{entries[index][:1000]}...\n\n{e}")
+                embedding_tensors += [torch.zeros(self.embedding_dimensions, device=device)]
+        return torch.stack(embedding_tensors)
+
+